Ape index, or ape factor, or gorilla index is a measure of the ratio of an individual's arm span relative to their height. A typical ratio is 1, as identified by the Roman writer, architect and engineer Vitruvius prior to 15 BC. Vitruvius noted that a "well made man" has an arm span equal to his or her height, as exemplified in Leonardo da Vinci's drawing, the "Vitruvian Man". In rock climbing it is believed that an Ape Index greater than one, where the arm span is greater than the height, provides for a competitive advantage, and some climbers have expressed the belief that exercise can result in an improved ratio.
The ape index is usually defined as the ratio of arm span to height. However, an alternative approach is arm span minus height with the result being positive, 0 or negative. Unlike the unitless ratio, this calculation produces a numeric value in the units of measurement used to represent the height and arm span.
Statistical significance in rock climbing
Multiple studies have been conducted into the effect of physiological factors, such as anthropometry and flexibility, in determining rock climbing ability. A number of these have included the Ape Index as one of the variables. However, the results have been mixed.
One study found that "untrainable" physical factors, including the Ape Index, were not necessarily predictors of climbing ability, (although they did note the significance of grip strength and percentage of body fat), in spite of a general tendency identified in previous studies for elite athletes in the sport to share these characteristics. This was supported by a later study that also found that the Ape Index was not statistically relevant. However, the authors of this second study noted that the findings may have been due to the low variability in the index between the climbers, who all had significantly higher Ape Indices than those found in the control group. Thus they left open the possibility that the Ape Index may be more significant when there is a greater degree of equivalence between the other traits under consideration.
Countering these studies are other works that have identified the Ape Index as a significant (or potentially significant) factor. A 2001 study comparing teenage male and female rock climbers noted that performance differences between the genders could be explained by a number of factors, one of which was the lower Ape Index found in the female climbers. Similarly, in a later work it was found that the Ape Index was statistically significant, and thus determined that it was one of a number of variables that provided the highest diagnostic value in the prediction of climbing performance.
It has been noted that swimmers tend to have longer arms in relation to their body size, perhaps because longer arms provide a stronger means of propulsion and a shorter trunk (or torso) reduces drag in the water. A notable example is Michael Phelps whose arm span is 3 inches greater than his height, affording him an index of 1.04.
There is evidence to support that having a higher index will be beneficial to a soccer goalkeeper. It can also compensate for being shorter than the recommended norm for a professional goalkeeper. Iker Casillas and Jorge Campos are prime examples of shorter goalkeepers who possess a higher than average index.
Small ape indices can also be beneficial. For instance, in exercises such as the olympic lifts and the bench press, shorter arms mean that the weights do not have to be raised so high compared to someone with longer arms and a bigger index, so the smaller indexed lifter does less work.
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